Abstract
Abstract The Lake Victoria region in East Africa is a hot spot for intense convective storms that are responsible for the deaths of thousands of fishermen each year. The processes responsible for the initiation, development, and propagation of the storms are poorly understood and forecast skill is limited. Key processes for the life cycle of two storms are investigated using Met Office Unified Model convection-permitting simulations with 1.5 km horizontal grid spacing. The two cases are analyzed alongside a simulation of a period with no storms to assess the roles of the lake–land breeze, downslope mountain winds, prevailing large-scale winds, and moisture availability. While seasonal changes in large-scale moisture availability play a key role in storm development, the lake–land-breeze circulation is a major control on the initiation location, timing, and propagation of convection. In the dry season, opposing offshore winds form a bulge of moist air above the lake surface overnight that extends from the surface to ~1.5 km and may trigger storms in high CAPE/low CIN environments. Such a feature has not been explicitly observed or modeled in previous literature. Storms over land on the preceding day are shown to alter the local atmospheric moisture and circulation to promote storm formation over the lake. The variety of initiation processes and differing characteristics of just two storms analyzed here show that the mean diurnal cycle over Lake Victoria alone is inadequate to fully understand storm formation. Knowledge of daily changes in local-scale moisture variability and circulations are keys for skillful forecasts over the lake.
Highlights
Lake Victoria, in tropical East Africa (Fig. 1), is a hot spot for intense convective storms and lightning (Flohn and Fraedrich 1966; Virts et al 2013; AlbrechtDenotes content that is immediately available upon publication as open access.et al 2016)
To choose the storm case studies, periods when storm events occurred in both Global Precipitation Measurement (GPM) observations and the East Africa operational forecast model (UKMO 2017, unpublished data) were chosen, to increase the likelihood that the simulations would produce a storm of sufficient quality to analyze
The dry period case is taken from the middle of the 2015 JJAS dry season, during a period in which almost no rain fell over Lake Victoria
Summary
Lake Victoria, in tropical East Africa (Fig. 1), is a hot spot for intense convective storms and lightning (Flohn and Fraedrich 1966; Virts et al 2013; AlbrechtDenotes content that is immediately available upon publication as open access.et al 2016). Lake Victoria, in tropical East Africa (Fig. 1), is a hot spot for intense convective storms and lightning It is estimated that 30 million people live on the shores of Lake Victoria, of which ;3.5 million, including 200 000 fishermen, are dependent on the lake for their livelihoods (Semazzi 2011). MONTHLY WEATHER REVIEW lake every year (Semazzi 2011; Cannon et al 2014). Despite recent improvements in weather forecast products over East Africa, including the addition of convection-permitting (CP) modeling (Chamberlain et al 2014; Woodhams et al 2018), forecasting storms over Lake Victoria remains a major challenge. A basic process-based understanding of the initiation, development, and propagation of individual storms within the Lake Victoria basin region is lacking, as is an understanding of the factors impacting the intensity of the storms, in particular the rainfall and nearsurface winds
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